The present invention relates to an insulated multiwire electric cable having protected, solderable and non-heat-sealing conductors.
Some cables for aeronautics for example are formed of plies of electric conductors protected by a metal cover and insulated, either by a thermoplastic polymer sheath, or by polyimide tape which may be covered with a sheath of thermoplastic polymer.
The protected electric conductors of known cables are copper wires traditionally coated either with tin, or with silver, or with nickel.
For severe environmental conditions, copper conductors protected by a silver coating are preferably used for the following reasons:
Copper wires protected with tin are not suitable for insulation of the ply by means of a taped coating or a polymer sheath since temperatures of the order of 300.degree.-350.degree. C. causes heat-sealing between the strands of the ply.
This heat-sealing is due to the fact that the melting point of tin is 232.degree. C.
This heat-sealing leads to disadvantages for the cables obtained are not very pliable and, because of the migration of the tin, some zones of the surface of copper conductors are no longer protected by the tin, which leads to deficiencies in solderability and possibilities of corrosion.
Wires protected with nickel have the disadvantage of not being solderable and force the user to carry out crimping or chemical cleaning operations of the nickel for forming connections.
Wires protected with silver are very suitable but they have the disadvantage of being expensive on the one hand and, on the other hand, the protection against electrochemical corrosion of the copper is not ensured since silver does not provide cathodic protection.
It has been thought that lead protections, a metal whose melting point is 321.degree. C., would seem to be suitable for the cable having solderable and non-heat-sealing conductors, but it has proved that lead has insufficient solderability or brazability when used with a 60/40 tin lead filler alloy and that lead does not wet the copper very well, which does not allow electric wires to be obtained which are correctly protected with lead from the point of view of surface appearance and uniform lead distribution.
Lead alloys such as the one with 30% tin and 70% lead have the disadvantage of being no longer solderable after accelerated aging treatments for 96 hours in air and at a temperature of 155.degree. C. or for 4 hours at 100.degree. C. in steam, in accordance with the tests described in the French standard NFC 20 630 paragraph 2.5.
The loss of solderability is due particularly to copper diffusion in the alloy layer which leads to the formation of a type Cu.sub.3 Sn nonsolderable intermetal compound.
Alloys of the lead-silver type have the disadvantage of being sensitive to atmospheric humidity during prolonged storage, which causes a complete loss of solderability.
The result is that it is difficult to have in an alloy both good in solderability after aging for 96 hours at 155.degree. C. in air or for 4 hours at 100.degree. C. in steam, and good wetability by molten alloy for electric conductors.
And yet, for cables having solderable and non-heat-sealing conductors, it is necessary for the solderability to be good after accelerated aging, for its offers a facility of effecting soldering and brazing rapidly even if the storage time of the cables has been more or less long.
It is also necessary for the wetability at a temperature of the order of 35.degree. C. of the alloy to be excellent, if not the metal coatings are offset and the soldering or brazing is defective, the coating of the electric conductor with the alloy being carried out by the hot dip process at a temperature of the order of 350.degree. C.